The invention concerns a method in calendering of a paper web, wherein the paper web is passed through a calendering nip, which calendering nip is formed between a roll which has been provided with a soft face by means of a coating or in an equivalent way and a heatable hard roll, in which method the heatable roll is heated so as to plasticize the surface layer of the paper web that is placed at the side of the heatable roll, preferably so that the temperature of the surface layer in the calendering nip exceeds the glass-transition temperatures of the polymers contained in the paper, and in which method, before entering into the calendering nip, the paper web is moistened, so as to lower the glass-transition temperatures of the polymers contained in the paper, and pre-heated.
The invention also concerns a calender that makes use of the method, which calender comprises a calendering nip, which is formed between a roll that has been provided with a soft face by means of a coating or in an equivalent way and a heatable hard roll and through which calendering nip the paper web is arranged to run, the heatable roll being arranged to be heated so as to plasticize the surface layer of the paper web that is placed at the side of the heatable roll, preferably so that the temperature in the surface layer in the calendering nip exceeds the glass-transition temperatures of the polymers contained in the paper, and the calender comprising means for moistening and pre-heating of the paper web immediately before the calendering nip.
By means of calendering of paper, attempts are made to improve the quality values of paper that have already been achieved or, with a standard quality level, to achieve a higher running speed or a better bulk of the paper. The plasticity (readiness of moulding) of paper can be increased by raising the temperature or increasing the moisture content in the paper. A considerable change takes place in the plasticity of paper when the temperature of the polymers contained in the paper rises up to or beyond the so-called glass-transition temperature. Then, the paper can be moulded more readily than below the glass-transition temperature. An increased moisture content in paper lowers the glass-transition temperature. Most commonly, the paper web is heated in a calender nip by means of a heatable roll, a so-called thermo roll, and, in addition to this, possibly by means of steam treatment before the nip. The steam treatment also increases the moisture content in the paper, and thereby lowers the glass-transition temperature. However, there is the problem that the paper does not have time to be heated sufficiently in the nip and that the steam escapes into the environment before the calender.
Besides to the prior art described above, reference is also made, among other things, to the DE Patent 4,126,233, which concerns a method and a device in glazing of a paper web. In said method, the paper web is first heated by means of heat radiators so that, on the faces of the paper, a plasticizing temperature is reached, after which the paper web is passed through a pair of rolls that form a nip, wherein the paper web is pressed and cooled in the nip. Said method and device involve a number of drawbacks, so that their objectives are unlikely to be achieved. Firstly, this comes from the fact that the web is heated considerably before the nip, in which case the web has time to be cooled before it reaches the nip. This affects the paper web so that, immediately after heating, the surface temperatures of the paper indeed become very high, but, before the nip is reached, these surface temperatures are lowered and, correspondingly, the temperature in the interior of the paper web becomes higher. One of the most important objectives of the solution of the DE patent publication was expressly to make the surface temperatures of the paper rise to the plasticizing temperature and to prevent the lowering of said temperatures before the nip. In said DE publication, the cooling proper was supposed to be carried out rapidly expressly in the nip. These objectives are not achieved in the solution of said DE publication, as was ascertained above.
With respect to the prior art, reference is made further to the U.S. Pat. No. 5,033,373. In said paper, a calender of two nips is described for two-sided glazing of a paper web. In the following, just one of the calender nips will be examined, because it is, in principle, identical with the other nip. Thus, the nip is formed conventionally out of a heatable hard roll and a soft-faced roll which is in nip contact with the hard roll. Before entering into the nip, the paper web is cooled by means of a cooling device, and thereafter, after the web has been cooled, the side of the web that will reach contact with the hot nip roll is heated by means of a heating device, preferably a heat radiator or hot-air blowing. The purpose of this heating is to make the paper face as hot as possible even before it enters into the nip.
After the heating device, the web is passed over a guide roll, which guides the web into the nip. Said guide roll is placed in such a position in relation to the nip that the web reaches contact with the hot roll even before it enters into the nip. Further, the position of the guide roll is adjustable so that, by means of regulation of the position of the guide roll, it is possible to regulate the distance of contact between the web and the hot roll before the nip. As was already stated above, the object of the solution in accordance with said US patent is to make the web as hot as possible even before the nip.
This method and device, however, firstly involve the drawback that the heating of the web by means of a separate heating device is carried out considerably before the nip, in which case the web has time to be cooled before it enters into the nip or reaches contact with the hot roll. Also, this method and device involve a second drawback, i.e. that the heating before the nip is carried out by means of a heat radiator or hot-air blowing, as a result of which heating the web is dried before it enters into the nip. On the other hand, the drying of the web raises the glass-transition temperature of the paper. This again has the further consequence that the temperature of the hot roll in the calendering nip must be made very high in order that the glass-transition temperature could be produced in the paper web in the nip. Owing to the very high temperature, the constructions necessarily become complicated and expensive.
In the U.S. Pat. No. 5,163,364, a method is described for calendering of a paper or board web, wherein the calendering nip is a so-called extended nip which is formed between a roll and a belt that runs along a glide shoe. In the method of said paper, the web is passed into the nip when wet, but a pre-moistening proper of the web is, however, not described in the paper. Nor does the paper give any description of a pre-heating of the web before it enters into the calendering nip.
In the publications FI Patent 63,981 (equivalent to U.S. Pat. No. 4,370,923), FI Patent 72,552, and FI Pat. Appl. 900432 (equivalent to U.S. Pat. No. 5,020,469) different modes of calendering are described and discussed, wherein the paper web is moistened right before the web enters into the calendering nip. In the solutions in accordance with these papers, the moistening of the web is carried out by means of steam, but pre-heating of the paper web before the web enters into the calendering nip is not suggested.
In the FI Pat. Appl. 912666 (equivalent to U.S. Pat. No. 5,245,920), heating of a heatable roll in a calender to a temperature higher than the glass-transition temperature of the material to be calendered is described. Further, in said paper, pre-heating and pre-moistening of the paper web are described before the web enters into the nip. On the contrary, the mode of pre-heating of the web described in said paper differs substantially from the solution of the present invention, being more difficult to carry out than in the present invention.
In the publications of FI Pat. Appl. 914933 (equivalent to U.S. Pat. No. 4,945,654) and FI Pat. Appl. 882845 (equivalent to U.S. Pat. No. 4,786,529), methods are described wherein the web is moistened with steam before the calendering nip, but pre-heating of the web is not suggested in these papers.